SA-4 SSRF protections implemented.

backend/app/services/mastodon_oauth.py

@@ -7,19 +7,17 @@ import json
 from secrets import token_urlsafe
 from urllib.parse import urlencode
 from urllib.error import HTTPError
-from urllib.request import Request, urlopen
+from urllib.request import Request
 from uuid import uuid4
 
 from backend.app.core.config import settings
 from backend.app.database import get_connection
 from backend.app.services.secret_store import decrypt_secret, encrypt_secret
+from backend.app.services.mastodon_security import open_no_redirect, validate_public_instance
 
 
 def normalize_instance(instance: str) -> str:
-    value = instance.strip().rstrip('/')
-    if not value:
-        raise ValueError('Mastodon instance is required')
-    return value if '://' in value else f'https://{value}'
+    return validate_public_instance(instance)
 
 
 def post_form(url: str, values: dict) -> dict:
@@ -29,7 +27,7 @@ def post_form(url: str, values: dict) -> dict:
         headers={'Content-Type': 'application/x-www-form-urlencoded'},
         method='POST',
     )
-    with urlopen(request, timeout=10) as response:
+    with open_no_redirect(request, timeout=10) as response:
         return json.loads(response.read().decode('utf-8'))
 
 

backend/app/services/mastodon_security.py

@@ -0,0 +1,62 @@
+## Copyright © 2026 Olaf Kolkman
+## SPDX-License-Identifier: GPL-3.0-or-later
+
+import ipaddress
+import socket
+from urllib.parse import urlsplit
+from urllib.error import HTTPError
+from urllib.request import HTTPRedirectHandler, Request, build_opener
+
+
+class RejectRedirectHandler(HTTPRedirectHandler):
+    def redirect_request(self, request, file, code, msg, headers, newurl):
+        raise HTTPError(request.full_url, code, 'Redirects are not allowed', headers, None)
+
+
+NO_REDIRECT_OPENER = build_opener(RejectRedirectHandler)
+
+
+def _is_blocked_address(address: str) -> bool:
+    parsed = ipaddress.ip_address(address)
+    mapped = parsed.ipv4_mapped if isinstance(parsed, ipaddress.IPv6Address) else None
+    candidates = (parsed, mapped) if mapped else (parsed,)
+    return any(
+        candidate.is_loopback
+        or candidate.is_link_local
+        or candidate.is_private
+        or candidate.is_multicast
+        or candidate.is_unspecified
+        or candidate.is_reserved
+        for candidate in candidates
+    )
+
+
+def validate_public_instance(instance: str) -> str:
+    value = instance.strip().rstrip('/')
+    if not value:
+        raise ValueError('Mastodon instance is required')
+    if '://' not in value:
+        value = f'https://{value}'
+    parsed = urlsplit(value)
+    if parsed.scheme.lower() != 'https' or not parsed.hostname:
+        raise ValueError('Mastodon instance must be an HTTPS public hostname')
+    if parsed.username or parsed.password or parsed.query or parsed.fragment or parsed.path not in ('', '/'):
+        raise ValueError('Mastodon instance must be a hostname-only HTTPS URL')
+    try:
+        port = parsed.port
+    except ValueError as error:
+        raise ValueError('Mastodon instance has an invalid port') from error
+    if port not in (None, 443):
+        raise ValueError('Mastodon instance must use HTTPS port 443')
+    hostname = parsed.hostname.rstrip('.').lower()
+    try:
+        addresses = {result[4][0] for result in socket.getaddrinfo(hostname, port or 443, type=socket.SOCK_STREAM)}
+    except socket.gaierror as error:
+        raise ValueError('Mastodon instance hostname could not be resolved') from error
+    if not addresses or any(_is_blocked_address(address) for address in addresses):
+        raise ValueError('Mastodon instance must resolve only to public IP addresses')
+    return f'https://{hostname}'
+
+
+def open_no_redirect(request: Request, timeout: int = 10):
+    return NO_REDIRECT_OPENER.open(request, timeout=timeout)

backend/app/services/plugin_manager.py

@@ -5,10 +5,11 @@ import json
 import logging
 from urllib.error import HTTPError, URLError
 from urllib.parse import urlencode
-from urllib.request import Request, urlopen
+from urllib.request import Request
 
 from backend.app.plugins.base import BasePlugin
 from backend.app.services.secret_store import decrypt_secret
+from backend.app.services.mastodon_security import open_no_redirect, validate_public_instance
 
 DEFAULT_POST_PREFIX = 'From my #LinkLog: '
 logger = logging.getLogger(__name__)
@@ -50,9 +51,10 @@ class MastodonPlugin(BasePlugin):
                 config.update(json.loads(row['config']))
         config['access_token'] = decrypt_secret(config.get('access_token', ''))
 
-        instance = str(config.get('instance', '')).strip().rstrip('/')
-        if instance and '://' not in instance:
-            instance = f'https://{instance}'
+        try:
+            instance = validate_public_instance(str(config.get('instance', '')))
+        except ValueError as error:
+            return {'status': 'failed', 'plugin': self.name, 'reason': str(error)}
         access_token = str(config.get('access_token', '')).strip()
         if not instance or not access_token:
             logger.debug(
@@ -92,7 +94,7 @@ class MastodonPlugin(BasePlugin):
                 },
                 method='POST',
             )
-            with urlopen(request, timeout=5) as response:
+            with open_no_redirect(request, timeout=5) as response:
                 response_body = response.read().decode('utf-8')
                 logger.debug(
                     'Mastodon post response: endpoint=%s status=%s body_length=%d',
@@ -139,9 +141,10 @@ class MastodonPlugin(BasePlugin):
                 config.update(json.loads(row['config']))
         config['access_token'] = decrypt_secret(config.get('access_token', ''))
 
-        instance = str(config.get('instance', '')).strip().rstrip('/')
-        if instance and '://' not in instance:
-            instance = f'https://{instance}'
+        try:
+            instance = validate_public_instance(str(config.get('instance', '')))
+        except ValueError as error:
+            return {'status': 'failed', 'plugin': self.name, 'reason': str(error)}
         access_token = str(config.get('access_token', '')).strip()
         post_ids = event.get('mastodon_post_ids') or []
         if not post_ids and event.get('mastodon_post_id'):
@@ -156,7 +159,7 @@ class MastodonPlugin(BasePlugin):
                     headers={'Authorization': f'Bearer {access_token}', 'User-Agent': 'LinkLog/1.0'},
                     method='DELETE',
                 )
-                with urlopen(request, timeout=5) as response:
+                with open_no_redirect(request, timeout=5) as response:
                     response.read()
             return {'status': 'deleted', 'plugin': self.name, 'count': len(post_ids)}
         except HTTPError as error: